There are two dominant difficulties for high-power DPSS lasers in the art, particularly for CW, TEM00-mode operations. The first issue is the heavy thermal loading management. The second is the intense pumping. These two issues are especially acute for the laser materials of a low stimulated emission cross-section, or for a quasi-3-level system, such as Yb: YAG, Tm: YVO4, Tm,Ho:YAG, Er: glass and minor lasing lines in Nd:YAG and Nd: YVO4, as well as for the tunable laser material Cr: LiSAF with the wing-pumping.
Further, after many years development, the effective and efficient pumping configurations, the key technology for modest-to-high power DPSS lasers, are still very limited. Besides, there is no architecture available in the known prior art which can be used to build modest-to-high power UV DPSS lasers, or to attain high-power operations at the minor laser line, leading to red and blue visible lasers with modest-to-high CW output power.
Briefly stated, the present invention intends to develop effective approaches in order to solve these dominant difficulties. Particularly, novel multipass pumping approach and engineering design have been developed for, thin-disk lasers, slab lasers and rod lasers.
The intense multiply or multipass pumping is employed to generate a uniform pump distribution within laser media so as to minimize thermal aberration and provide high optical gain. They are characterized by using multipass and zig-zag pumping paths and confining pumping beams mainly via total-internal-reflection (TIR) to significantly reduce multiple reflection losses. They will readily allow pump power and pump density, and output power scaling to much higher levels. Their pumping intensity and efficiency are able to compare to the end-pumping scheme.
The corresponding engineering designs for the zig-zag slab laser in solving cooling problems and multiply pumping are completely different with traditional arrangement in the art. They provide stress-free and O-ring-free slab mounting and sealing. They enable an entire slab pumping and cooling from tip to tip with both direct and conductive cooling systems with a zig-zag optical path. They are highly cost-effective and compact. Meanwhile, they provide intense uniform multipass pumping and high-gain aberration-free laser slabs.
A zig-zag slab laser combined with a one-dimensional beam-expanding cavity would enable one to effectively solve thermal distortion problems, obtain TEM00-mode operation and maximize the energy extraction from a laser slab. Such combination also leads to achieving high-performances of intracavity harmonic generations and true CW operation over wide spectral ranges, from red to ultraviolet. Further, when some slab-shaped nonlinear crystals and/or Q-switch are inserted to the expanded mode portion of such a laser cavity, the high-power, high-energy operations in the infrared, visible or UV spectra range can be realized. The modest-to-high power UV DPSS laser would become a strong competitor for the eximer laser particularly when some appreciated nonlinear crystal for FHG is available.
Besides, based on achieving intense pumping and high-power TEM00-mode operations and with the aid of an additional approach for minimizing spatial hole burning, one is able to realize high-power laser operation at minor lasing lines, leading to generations of CW red and blue visible coherent light with modest-to-high levels of output power for laser display.
One of the key gains of going to diode pumping is reducing thermal loads on the laser rods and the concomitant thermal distortion and fracture. However, thermal loading is still a big area of concern for high power DPSS lasers. In the high-power pumping condition a laser material itself is optically and mechanically distorted by power dissipated as heat. The thermal distortions severely prevent the laser from efficient operation in the TEM00 mode. Even for side-pumped lasers, energy distortion can be quite uneven due to the deviation from uniform pump distribution and thermal distortion remains a problem.
In the prior art, the technique with two identically pumped laser rods is commonly used to compensate the birefringence and bifocusing. However, stable operation only can be obtained within a certain pump-power range. It is difficult to correct in a lumped laser rod and to provide massive products. Besides, the disk laser developed by Germany has a complex pump approach and some practical limitations in the cavity arrangements.
A properly designed slab laser with zig-zag path is well suited for reducing undesired thermo-optic effects. However, the related critical design issues and engineering problems are extremely tough in the art. After two decades of development work, the laser community has still not accepted the slab laser. The reasons for that are a number of practical engineering problems which have prevented so far the realization of the potential advantages of the zig-zag slab laser over the rod geometry.
In general, the first problem is low efficiency. The awkward rectangular beam geometry which requires conversion to a circular beam in useful optical applications, combined with the fact that the full width of the slab can usually not be used because of distortions at the edges. The second one is residual distortions. Deformation of the ends and pump faces of the slab due to thermal strain have resulted in beam qualities considerably below expectations. The third problem is high fabrication cost. The very demanding and therefore expensive laser head design are resulted from the complicated mechanical mounting and sealing geometry, and stringent pumping and cooling requirements.
In the prior art, side-pumping geometry puts very stringent requirements on the diode wavelength tolerance with the use of regular laser materials because of the short absorption lengths available. The absorption efficiency is seriously limited. Beam quality can also be negatively impacted because of axially asymmetric gain profiles that can be imprinted on the output beams.
Meanwhile, deviation from uniform pump distribution causes thermally induced optical aberration in laser material. The transmitted and aberration wave front from laser material induces diffraction loss in the resonator. The optical efficiency of the laser is essentially determined by the ratio of round-trip optical loss to optical gain. Compared with the increment of laser gain, proportional to the pump power, the diffraction loss, caused by thermally induced aberration increases more rapidly as the pump power increases. The optical efficiency decreases relative to the pump power, and consequently the output power is limited.
The multipass pumping approach can be employed to obtain uniform pump distribution in laser materials. However, it is vital in practice to obtain a high reflectivity in order to ensure the successful multi-reflecting pump process. In this process, the pumping beams are multiply reflected and multipass through the gain medium repetitively until they are completely absorbed. The angles of incidence of the pumping beams within the pump cavity usually vary and spread widely. And the pumping beams commonly need more than tens or a few tens of reflections for pumping. With conventional multilayer dielectric coatings, the reflectance is very sensitive to the incident angle. Meanwhile, the reflectivity of the metallic coatings such as gold or silver drops dramatically at large incident angles.